1. Field of the Invention
The present invention relates to an antenna device operable in multiple frequency bands, and in particular to an antenna device which may be built into a radio apparatus.
2. Description of the Related Art
There is a trend that mobile phones or personal computers (PCs) with radio capability have multiple purposes and multiple functions. The above trend requires an antenna device which may be operable in multiple frequency bands or in a broad frequency range.
For the above requirement, e.g., the applicant applied for and obtained a patent on an invention of a built-in antenna of a radio apparatus which is operable in multiple frequency bands having impedance that may be smoothly matched, as disclosed in Japanese Patent Publication (Toroku), No. 3775795.
In addition, a conventional antenna device configured to be operable in multiple frequency bands or in a broad frequency range is disclosed in Japanese Patent Publication of Unexamined Applications (Kokai), No. 2002-64324.
More specifically, as shown in FIG. 10 and in paragraphs 0096-0102, the antenna device disclosed in JP 2002-64324 includes a planar-shaped microstrip antenna 42 and a monopole antenna 1. The microstrip antenna 42 is arranged parallel to a ground plane 6. An end of the microstrip antenna 42 is connected to an end of the monopole antenna 1. The antenna device disclosed in JP 2002-64324 has a single resonant frequency, and the monopole antenna 1 is about half as long as a wavelength of the resonant frequency.
The planar-shaped microstrip antenna 42 has a length “a” and a width “b”. The length “a” is about half as long as the wavelength of the resonant frequency. It is described in JP 2002-64324 that a greater value of the width “b” produces a greater value of an antenna's electrical volume, and contributes to a broader frequency range thereby.
Another conventional antenna device is disclosed in Japanese Patent Publication of Unexamined Applications (Kokai), No. 2005-94501. More specifically, as shown in FIG. 5 and in paragraphs 0021, 0022 and 0031-0033, the antenna device disclosed in JP 2005-94501 is a planar multiple-layered antenna including a rectangular conductive pattern 43 and a U-shaped conductive pattern 45. The rectangular conductive pattern 43 and a ground board conductor 49 are located on a same plane.
It is described in JP 2005-94501 that the planar multiple-layered antenna is resonant at multiple frequencies, having a first resonant frequency f1 and a second resonant frequency f2, where f1<f2. At the first resonant frequency f1, antenna current resonance occurs on the U-shaped conductive pattern 45 as a whole. At the second resonant frequency f2, resonance occurs along an inner portion of the U-shaped conductive pattern.
The above built-in antenna of a radio apparatus disclosed in JP 3775795 includes a first antenna element being folded and having a grounded end which may be resonant at a relatively lower frequency, and a second antenna element having an open end which may be resonant at a relatively higher frequency. The antenna disclosed in JP 3775795 is configured to allow impedance of the second antenna element to be matched by adjusting a position where a forward path and a backward path, both of the folded first antenna element, are short-circuited.
As the resonant frequency of the second antenna element becomes higher, the above short-circuit position shall be located closer to a feeding point for impedance matching, making the impedance more inductive at the resonant frequency of the first antenna element. Thus, it may be difficult in some cases to determine each of the resonant frequencies independently.
Meanwhile, nothing is disclosed in JP 2002-64324 with respect to multiple-frequency resonance of the antenna device. As the antenna device disclosed in JP 2005-94501 utilizes the resonance on the U-shape as a whole and the resonance along the inner portion of the U-shape, it may be difficult to separate the resonant frequencies beyond a certain extent or to determine each of the resonant frequencies independently.